The present invention relates to an air-fuel ratio closed-loop control method and apparatus for an internal combustion engine.
It is a well-known practice to provide an internal combustion engine with an air-fuel ratio closed-loop control system. Such a system calculates a proportional plus integral value of a detection signal fed from a concentration sensor to form an air-fuel ratio (A/F) correction factor. The sensor detects the concentration of a particular component contained in the exhaust gas. An example of such a sensor is an oxygen (O.sub.2) sensor for detecting the concentration of oxygen in the exhaust gas. The air-fuel ratio closed-loop control system corrects the feeding rate of the fuel injected into the engine according to the calculated A/F correction factor so as to control the air-fuel ratio at the desired value.
According to conventional air-fuel ratio closed-loop control, however, as the proportion constant and the integration time-constant for calculating the proportional plus integral value of the detection signal from the O.sub.2 sensor are fixed predetermined constants, it is very difficult to always execute optimum air-fuel ratio closed-loop control irrespective of the change in the engine-operating condition. For example, when the engine is fully warmed-up, the air-fuel ratio should be more quickly controlled than when the engine is being warm-up so as to improve the emission control characteristics and engine response. However, according to prior air-fuel ratio control, the control speed of the closed-loop is always maintained at a constant.